Dams in the Amazon river basin have induced confrontations among developers, governmental officials, indigenous populations and environmentalists. Amazonian hydroelectric dams are usually justified on the basis that they supply the energy needed for economic development in a renewable form that also minimizes carbon emissions. Recent scientific reviews have considered the environmental impacts of damming Amazonian rivers 1–3, but regrettably, the effects of dams have been assessed mainly through studies undertaken only in the vicinity of each dam 4. Such a local approach generally ignores the far larger, basin-scale, geomorphological, ecological and political dimensions that will determine the future productive and environmental condition of the river system as a whole. For networks of large dams on huge rivers 5, far less consideration has been given to the need to assess environmental impacts at regional to continental scales.